The present invention relates to a converter for converting a primary voltage supplied to a primary port to a secondary voltage supplied to a secondary port, the converter comprising a primary switching device connected in series with the primary port, a secondary switching device connected in series with the secondary port, primary switching control means for controlling the switching of the primary switching device, and secondary switching control means for controlling the switching of the secondary switching device. The converter may be used in AC-DC and DC-DC power conversion, in particular in the lower power range such as for, but not restricted to, universal power supplies. The invention also relates to a control method for such a converter, and to a control circuit for implementing the control method. The invention further relates to a display apparatus comprising such a converter.
In converter applications which have to deal with a wide input voltage variation, for example, in a range of 50-400 V, the conduction time of the primary switching device at the primary port of the converter has to be chosen in such a way that an energy transfer which is high enough is ensured, even for the lowest primary voltage. If a fixed conduction time of the primary switching device is used, as e.g. disclosed in EP-A-0,336,725, extremely high currents result for higher primary voltages, leading to high current ratings, high RMS losses, and to a large amount of circulating energy in the converter.
As an example, a bidirectional flyback converter is described in more detail. U.S. Pat. No. 3,986,097 and EP-A-0,013,332 disclose power train structures of a bidirectional flyback converter.
In general, a bidirectional flyback converter comprises a transformer having a primary winding and a secondary winding, a primary switching device coupled in series with the primary transformer winding, and a secondary switching device coupled in series with the secondary transformer winding. If it is assumed that all circuit elements are ideal (no parasitic, leakage or stray inductances, no parasitic capacitances), the operation of the converter basically has two modes: a conduction mode at the primary side of the transformer and a conduction mode at the secondary side of the transformer. Both modes can be divided into two submodes: during a first time period of each conduction mode, energy is delivered to the associated side, while during a second time period of each conduction mode, energy to be delivered to the other side is stored in the transformer. During said first time period, the current can flow either through a diode coupled in parallel with the switching device, or through the switching device itself if it is bi-directional. The switching device can be switched on during said first time period under zero voltage condition.
In a bi-directional flyback converter, power may flow from a primary side to a secondary side but also in the reverse direction. The secondary side may be controlled to reflect excessive power to the primary side, which allows for a regulation of an output voltage or an output current at the secondary side. The primary side may be controlled to provide enough energy to the secondary side.
Prior art flyback converters suffer from one or more of the following problems: high switching losses, necessity of an isolated feedback (at least when an accurate control is mandatory), difficult no-load control, and low efficiency due to losses in the output rectifier in the case of low output voltages.
It is an object of the invention to lower the switching losses for a converter, allowing for higher switching frequencies and/or smaller designs.
According to the present invention, this object is achieved in a converter control method which is characterized as defined in claim 1, a converter which is characterized as defined in claim 5, and a control circuit for a converter which is characterized as defined in to claim 13.
In the converter according to the invention, variations of the input voltage are controlled at the primary switching device, while load regulation takes place at the secondary switching device.
The control according to the invention allows for a relatively low amount of circulating energy that results in a good partial load efficiency of the converter. In the case of no-load, the power dissipation can be made small at a finite switching frequency.
Under steady state of the bi-directional converter, a certain minimum amount of energy is reflected back to the primary side. Under no-load condition the secondary side controller will reflect the complete energy (minus the energy dissipated during one cycle through the transformer) transferred from the primary. The energy transfer from and to the secondary side needs a certain time span. Thus the converter is regulated at a finite frequency. The primary side control in accordance with the invention adjusts the amount of excessive energy by adapting the on-time in dependence of the input voltage. This results in a better part load efficiency for input voltages.
A further aspect of the invention provides a display apparatus comprising a converter as defined in claim 13.